Oscillator frequency control system



United States Patent O 3,109,148 GSClLlJA'lUR FREQUENCY CONTRL SYSTEM iviarvyn il. Siegel, Haddonfieid, Nall., assigner, by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Delaware Filed Apr. 23, 1959, Ser. No. @3,3% 3 Claims. tl. SSL-145) This invention relates to frequency control systems and especially to frequency control systems which employ frequency discriminating circuits.

lt is ioften desirable in communication systems, navigation systems, and measuring systems -to provide an oseillatory wave whose frequency stability is not subject to substantial changes. For example, in the FM telemetering art the output wave of an FM transmitter having center `or nominal frequency is deviated to correspond to the data to be sent. To avoid mransmission of erroneous information it is important lthat the center frequency of the transmitter be maintained constant. ln ythe UHF art, the prior ant taught that the output signal of a magnetron, for example, could bei mixed `with a reference frequency wave consisting of the output signal of a crystal oscillator which has been multiplied in frequency. The mixer produced a difference frequency signal which was applied tot a frequency discriininator which theoretically had a cross-over frequency (le, the frequency at which it produced Zero output signal), which was the same as that of the difference frequency when the magnetron was operating at its center frequency. The discriminator would produce: a DC. error signal for stabilizing the oscillator, whose polarity and amplitude were a function of the deviation, if any, of the difference frequency signal from its frequency when the magnetron was operating at its center frequency and indicated the sense and the magnitude of any deviation of the magnetron frequency therefrom.

ln practice, however, the actual cross-over frequency of a discriminator `in operation is subject to change bccause of changes in the parameters `of its circuit componente. Hence the characteristic t2-curve of' the discriminator may shift either (to the left or right of the origin so that, in the prior art system, a zero output signal was produced by `the discriminator at a frequency other than the desired frequency. Funthermore, due to the shift, an erroneous component was introduced in the output signal of the discrimina-ttor, not only ywhen the frequency of the input signal was lthe sante as the nominal cross-over frequency, but also at other input signal frequencies. Consequently, the discriminator would produce an error signal which itself inaccurately represented the deviation of the magnetron output signal from the desired center frequency so would not tend to restore the magnetron to `the desired center frequency. ln` sho-rt the precision of these prior systems twas no better than the precision of the discriminator.

Another disadvantage of the prior art system was that since the output of a discriminator is essentially DE., D C. amplifiers with their attendant problems of drift, would ordinarily be used, introducing even more error into the control system.

lt is therefore an object of the present invention to provide an oscillator frequency controi system whose accuracy and eiilcieney is substantially una fed by changing parameters of some of the frequency-sensitive: circuits connected therewith.

Another object of the invention is 'to provide a novel system which employs a` discriminator for controlling the frequency of an oscillator, wherein changes in the crossover frequency of the diseriminator do not appreciably impair the accurate operation of the system.

Still another object of the invention is to provide a 2 frequency control system employing a discriminator but permitting A C. amplification of the control signal produced thereby.

t is a further object of the invention to provide a frequency control system for holding precisely the frequency of a UHF oscillator, such as `a magnetron, to a harmonic of a crystal-controlled oscillator substantially independently of variations in the parameters of the frequency control system.

According to my invention a reference: signal having a frequency which is a subharmonic of the `controlled oscillators center frequency is multiplied and mixed with the output signal of the controlled oscillator to produce a difference signal having substantially the same frequency as the frequency of the reference frequency signal. Portionsof the reference frequency signal and the difference frequency signal are then alternately supplied at an audio rate, via a switching circuit responsive to an audio switching frequency signal, to a conventional disoriminator. The discriininator produces an output signal that contains AC. component which is applied, together with the switching signal, to a synchronous detector which produces a DC. error signal whose polarity indicates the sense of any deviation in the frequency of the controlled oscillators actual output and whose amplitude indicates Ithe magnitude thereof. This error signal may then be employed to reduce the frequency deviation (from :the center frequency) of the signal produced by the controlled oscillator.

Referring to FIGURE l an oscillator l@ is shown which is intended to be controlled so that the frequency f,L of its actual output signal at C remains substantially at a predetermined center frequency fo. waveforms at the lettered points on FlG. l are shown designated by the same letters in the various parts of FIG. 2. The lpresent invention will be explained in connection with a system in which the oscillator lll) is a UHF oscillator such as a magnetron or klystron although it is equally linaplicable to many other types of oscillators. The oscillator lil may comprise, for example, a voltage-tunable magnetron such as the GE Z-5225 having an external cavity, or a planar triode such as the 2G39. The voltage-tunable magnetron was originally designed as a wideband oscillator having a high frequency-deviation sensibility Vwhich may be on the order of l megacycle per volt. ln cases ywhere the magnetron is to tbe used for FM telemetering purposes, for example, it is desired to con-trol it so that its actual frequency fa deviates `as little as possible from its center frequency fo to prevent false telemetering information from being transmitted.

A reference frequency signal, fr, is provided by source ll which may be, for example, Va ZNSGQ transistor or a Sli-l0() transistor used in a conventional transistorized oscillator circuit such `as is shown in PIG. 1-227 on page 346 of Handbook of Piezo-Electric Crystals for Radio Equipment Designers (October i956) published as WADC Technical Report 56456 and also known as ASHA Document No. ADEMAS. The reference 'frequency signal fr at A is applied to `a `frequency multiplier l2 which may consist of a conventional class C amplifier having a` tuned. output circuit tuned to a harmonic of the input signal. Thus, for example, if fr 'is 78 mc. and the multiplier l2 multiplies it by a factor (n) of 8, the `output signal nf', at B will 'be a 624 rnc. signal. The signal nfl. is then applied to one .input of :a conventional non-linear mixer l5 which `may 'be a 1N263 crystal diode, for example, which generates harmonics of the input signais thereto. lf a wave at 2496 rnc., which is the fourth harmonic of nfm, ie., mnfr where m24, is selected by a. tuned circuit and mixe-d Iwith the actual signal fa at C, the loutput of the mixer i3 will include a lilterahle signal havingthe difference frequency fdt'fdzmufr-fa).

rl`hus, if fa is 2574 mc. the `difference frequency signal fd will be 78 mc., the frequency of fr. When the frequency of the actual signal fa equals the center frequency fo, fd at D equals fr, but when fa deviates from the center frequency fo, the difference signal fd will not be the same as fr. It should be appreciated, however, that all of the frequency multiplication may be performed in the multiplier l2 rather than part therein :and part in the mixer 13.

In accordance with my invention, shown in the dashedline box lo", the reference frequency signal fr is applied to the contact lo of a switch indicated generally `and schematically at 2G. The arm 3G of Athe switch normally is in contact with contact ifi. The `difference signal fd produced `by the mixer i3 is applied to the contact 14. A source 22 of a switching signal having a frequency fs (where fs is in the audio range and substantially below the lowest frequency, if any, at which the output of C of the oscillator i@ is to be modulated) is applied across E to the winding 25 of the switch 2i) causing the arm 3@ to make Contact with `contact lo whenever the signal at E exceeds a certain level. in one embodiment of the invention a switching signal of 60 c.p.s. was employed. ln actual practice the switch Ztl may comprise a conventional electronic switching circuit having two gate tubes or transistors whose `output circuit is common. To one of the tubes fr and audio gating pulses `at fs (at Zero phase) from source 22 are applied. To the other tube, fd and gating pulses fs l80 out of phase with the first set of pulses are applied. The gating pulses themselves may be derived from rectified and clipped sine Waves produced -by conventional circuits, `or may originate from pulse-producing circuits such `as multivibrators.

As a result of the action of switch 2u the reference frequency signal fr and the difference frequency signal fd are alternately applied as a composite wave to the input (see F) of the amplifier and limiter 35 where any amplitude variations therein are removed prior to application thereof to a conventional disoriminator 40.

`The discriminator liti, which may be a Foster-Seeley type or a slope detector, is constructed so that its crossover frequency is nominally the same as that of the reference signal fr. It will first be assumed that the actual cross-over frequency of the discriminato-r remains at the frequency fr, i.e., 78 mc. When the composite signal, as amplified and limited, is applied to the discriminato-r 4t) the latter will produce an output signal at G consisting of a train of rectangular pulses whose amplitude will be a function of the magnitude of the `difference in the frequencies of fr and fd applied thereto, which of course will depend on the extent of the deviation of the signal fa at C from fo. The polarity of the signal at G will depend on the sense of the deviation, i.e., whether fd is higher or lower than fr (depending whether fa is higher or lower than fo).

The A.C. discriminator output signal iat G is then coupled to a conventional `audio amplifier 45 which is constructed to have a bandwidth sufficient to pass, preferably, up -to the tenth harmonic of the signal fs so that no phase shift at the switching frequency is introduced. As there is capacitive coupling in the amplifier 4S its output signal at H will consist of an A.C. component (which does not any `longer indicate, in itself, the sense of the deviation) which is then applied, by transformer coupling, for example, to a conventional synchronous detector to which the switching signal fs from `source 22 Vis also applied.

The synchronous detector 50 may be of the type shown in FIGURE 5.8-2 `on page 181 of volume II of Servomechanisms and Regulating System Design (Wiley, 1955) by Chestnut and Mayer. The synchronous detector Si) produces an error signal at I consisting essentially of a DC. signal whose polarity indicates the direction of the `deviation of the controlled oscillators output signal and whose amplitude indicates the extent of the .deviation This signal is applied to a conventional frequency control system indicated schematically in block 55'. The lsystem 55 may consist of a reactance tube or the like in cases wher-e the oscillator 10 is not so constructed as to be capable of `being deviated by an error signal applied directly thereto. ln the case of voltagetunable magnetrons, however, the frequency control sys- `tem S5 may, as is quite conventional, 'be part yof the UHF generator it) itself, eg., an electrode whose potential, changed by the application of the error signal thereto, produces frequency changes in the oscillator.

ln 4the foregoing it has been assumed that there is no change or shift in the cross-over lfrequency of the discriminator 4t), and it has been seen that the accuracy of the overall ycontrol system of FIG. 1 is limited essentially only by the toleran-ces of the frequency stability of the reference `oscillator 11. However, it is an important feature of my invention that even if the cnoss-over frequency of the ydiscriminator 4i) shifts, no frequency error is introduced thereby. That this is so may be appreciated from the following explanation.

it will be assumed that the cross-over frequency of the disoriminator does shift to the left, ie., it produces Yero output not at fr but at some lower frequency. Consequently, the output wave of the discriminator 4i) will be as shown in G of FIG. 2. As seen in G', the reference frequency component fr of the composite wave (see F) applied to the d-iscriminator will now cause, not a zero output signal therefrom (as shown in waveform G in the previous case), but a positive pulse Gti, whereas the signal fd will cause the production of positive pulse 62, since both input signals will fall effectively at higher points on the S-curve of the disoriminator. However, if Vthe S-curve is linear the difference in the peaks of pulses Gti and 62 (-i.e., the A.C. component) will be the same as was shown in G; the reason for the differences in G and G arise solely from the addition of a D.C. component due to the dnift of the cross-over frequency. When the wave G is amplified in -audio amplier 4S, only the A.C. ccmponenft will be amplified to produce the wave H', which is identical to H, and hence the output wave l of the synchronous detector 50 in this case will be the same as in the case whene no cross-over drift occurred.

in the event that the cross-over frequency shift is to the right, the bursts of the composite wave which correspond to fr will cause a negative-going pulse to be produced by the discriminator whereas the burst of fd will cause a negative pulse but of smaller amplitude. However, after passage through the amplifier 45, only the A.C. component will remain and the synchronous detector Sil will convert this into a DC. wave having the correct polarity and magnitude to reduce the deviation of the oscillator by the proper amount, notwithstanding the drift of the cross-over frequency.

I claim:

.1. A system for controlling the frequency of an oscillator intended to produce an output signal at a frequency f, said frequency f having a predetermined nominal value fo, said system comprising a reference signal source providing at a first output a reference signal at frequency f1. and at a second output a reference signal at frequency ufr, where f,.=f/n, a and n are both integers, and a has a value less than n, heter-odyne mixer means coupled to said oscillator and said second output of `said reference signal source and responsive to said reference signal at frequency ufr and the output signal of said oscillator for providing a iirst heterodyne signal at frequency Plr where b is an integer having a value such that frequency discriminating means, switch means coupied to said first loutput of said reference signal source, the outr1 il put of said lieterodyne mixer means and the input of said frequency discriminating means, a control signal source coupled to said switch means for providing an alternating control signal to said switch means, :said switch means being responsive to said alternating con-trol signal to connect, alternately at the frequency of said control signal, said first output of said reference signal source and the output of said heterodyne mixer means to the input of said frequency discriminating means, synchronous detector means, aud-io amplifier means coupling the output of said frequency discriminating means to one input of said synchronous detector means, said audio amplifier means passing only the alternating current components of the signal at the output of said frequency discriminating means, means coupling the output of said control signal source to a second input of said synchronous detector means, and frequency control means coupled to said oscillator and responsive to the output of said synchronous detector means for controlling the frequency of the output signal of said oscillator.

2d A frequency controlling system as in claim l Wherein a=(n-1) and 11:1.

3. A system for controlling the frequency of a first oscillator intended to produce an output signal at fren quency f, sm'd frequency f having a predetermined nominal value fo, said system comprising a reference oscillator for providing a reference signal having a stable frequency fr, where f,.=f/n, n being `an integer greater than 2, frequency multiplier means coupled to the output of said reference oscillator, said frequency multiplier means being adapted to provide an output signal at a frequency mfr, Where m=(1z*1), heterodyne mixer means coupled to the outputs of said frequency multiplier means and said first oscillator, said heterodyne mixer means being constructed and arranged to provide an output: signal having a frequency equal to f-mf frequency discriminating means, switch means `coupled to said `reference oscillator, said heterodyne mixer means and said frequency discriminating means, a control signal source coupled to said switch means for providing an alternating contro-l signal, said switch means being respons-ive to said alternating control signal to connect, alternately at the frequency of said control signal, the output of said reference oscillator and the output of said hetenodyne mixer means to the input of said frequency discriminating means, synchronous detector means, audio amplifier means coupling the output of said frequency discriminator means to one input of said synchronous detector means, said audio amplifier means passing only the alternating current components of the signal at the output of said frequency discriminating means, means coupling said control signal source to a second input of said synchronous detector means, and frequency control means coupled to said first oscillator and responsive to the output of said synchronous detector means for controlling the frequency of said rst oscillator in accordance with the signal supplied by said synchronous detector means.

References Cited in the file of this patent UNITED STATES PATENTS 2,424,833 Korman July 29, 1947 2,640,155 Rambo May 26, 1953 2,838,673 Fernsler et al. June lllJ 1958 

1. A SYSTEM FOR CONTROLLING THE FREQUENCY OF AN OSCILLATOR INTENDED TO PRODUCE AN OUTPUT SIGNAL AT A FREQUENCY F, SAID FREQUENCY F HAVING A PREDETERMINED NOMINAL VALUE FO, SAID SYSTEM COMPRISING A REFERENCE SIGNAL SOURCE PROVIDING AT A FIRST OUTPUT A REFERENCE SIGNAL AT FREQUENCY FR AND AT A SECOND OUTPUT A REFERENCE SIGNAL AT FREQUENCY AFR, WHERE FR=FO/N, A AND N ARE BOTH INTEGERS, AND A HAS A VALUE LESS THAN N, HETERODYNE MIXER MEANS COUPLED TO SAID OSCILLATOR AND SAID SECOND OUTPUT OF SAID REFERENCE SIGNAL SOURCE AND RESPONSIVE TO SAID REFERENCE SIGNAL AT FREQUENCY AFR AND THE OUTPUT SIGNAL OF SAID OSCILLATOR FOR PROVIDING A FIRST HETERODYNE SIGNAL AT FREQUENCY 